In the last decades, ionization current diagnostics has proven itself to be an effective approach in investigating the mechanism of fuel combustion and optimizing spark-ignition (SI) engine control.
Monitoring of ionization current waveform in SI engines fueled with gasoline or different fuels and sophisticated calibrations are currently used for online diagnosis of misfirings and knock detection, cam phase determination, air/fuel ratio estimation, cylinder pressure estimation and peak cylinder pressure position estimation [1-5].
Most SI engines function with an inductive ignition system. However, large ignition discharge currents can mask ionization current at the beginning of propagation of the combustion (during a so-called front flame phase of the combustion process). During initial flame propagation the ionization current that is significantly masked by the spark discharge current remains hardly detectable if the discharge current of the inductance of the spark plug circuit persists long after ignition starts. How to discriminate the interference of ignition discharge current from the ionization current has been and a key issue for a long time in the ionization current measurement technical field.
Filtration by the so-called “Blind Source Separation” (BSS) method of monitored current, in which the independent original signal can be extracted from the statistically independent source signals, may be a way to discriminate the ignition spark discharge current from ionization current, however the effort may be ineffective in case of significantly corrupted current signals. In any case for achieving reliable analytic diagnosis and/or indirect assessment of important characteristics of the combustion process complex independent component analysis methods should be used [1].
It is observed that high frequency components of the current may hardly circulate both in the primary or secondary windings of the ignition coil, thus they tend to flow through parasitic capacitances towards the supply node of the first spark electrode and then to ground and/or are dissipated as waste heat in the magnetic core of the ignition coil. Therefore, potentially useful information derivable from sensed current data gathered during the crucial ignition and the flame-front phases remains unexploited.
Detection of the low frequency current circulating in the primary circuit of the HV coil is an approach followed in known Delphi and Bosch systems, however the approach allows only the detection of the post spark ion current and requires the use of auxiliary components as HV diodes, capacitors, resistors and/or a DC supply [2].
Reportedly, monitoring of the ionization current “during initial spark phases” has been achieved with additional electrodes inside the combustion chamber.
In the prior Italian patent application No. MI2001A001896, the present applicants disclosed an effective device and circuit arrangement for significantly reducing shortcomings of traditional sensing schemes and related hardware to sense ionization current (sometimes called ion current) during the first two phases, namely the ignition phase and the flame-front phase.
As described in the above cited prior application, the problem is alleviated by providing a resistive element connected to the ground electrode of the spark plug, such that when the spark plug is mounted in a SI engine combustion chamber, the ground electrode of the spark plug becomes electrically connected to the engine body through a resistive element interposed therebetween in the flow path of the ionization current. Moreover, according to a disclosed embodiment, the ground electrode of the spark plug is provided with an appendix adapted to constitute an accessible current sense terminal outside the combustion chamber or wired to it. In this way, it is made possible to detect with enhanced discrimination the ionization current even during the ignition and flame-front phases by sensing the voltage between the integrated sensing terminal and the ground node constituted by the engine body. The whole content of the above cited Italian patent application is incorporated herein by express reference.